diff --git a/common/enum_base.h b/common/enum_base.h
index f4c805cc576a..03fdb9e79b5f 100644
--- a/common/enum_base.h
+++ b/common/enum_base.h
@@ -210,7 +210,7 @@ class EnumBase : public Printable<DerivedT> {
 // Use this immediately after the Carbon enum class body to define each named
 // constant.
 #define CARBON_ENUM_CONSTANT_DEFINITION(EnumClassName, Name) \
-  constexpr EnumClassName EnumClassName::Name =              \
+  inline constexpr EnumClassName EnumClassName::Name =       \
       EnumClassName::Make(RawEnumType::Name);
 
 // Use this in the `.cpp` file for an enum class to start the definition of the
diff --git a/common/raw_hashtable.h b/common/raw_hashtable.h
index ae642a615f63..613990af72ae 100644
--- a/common/raw_hashtable.h
+++ b/common/raw_hashtable.h
@@ -147,7 +147,7 @@ namespace Carbon::RawHashtable {
 
 // If allocating storage, allocate a minimum of one cacheline of group metadata
 // or a minimum of one group, whichever is larger.
-constexpr ssize_t MinAllocatedSize = std::max<ssize_t>(64, MaxGroupSize);
+inline constexpr ssize_t MinAllocatedSize = std::max<ssize_t>(64, MaxGroupSize);
 
 // An entry in the hashtable storage of a `KeyT` and `ValueT` object.
 //
diff --git a/common/raw_hashtable_metadata_group.h b/common/raw_hashtable_metadata_group.h
index 1d87d879b4f6..48bd11fb9a16 100644
--- a/common/raw_hashtable_metadata_group.h
+++ b/common/raw_hashtable_metadata_group.h
@@ -43,7 +43,7 @@ namespace Carbon::RawHashtable {
 // We define a constant max group size. The particular group size used in
 // practice may vary, but we want to have some upper bound used to ensure
 // memory allocation is done consistently across different architectures.
-constexpr ssize_t MaxGroupSize = 16;
+inline constexpr ssize_t MaxGroupSize = 16;
 
 // This takes a collection of bits representing the results of looking for a
 // particular tag in this metadata group and determines the first position with
diff --git a/toolchain/base/int.h b/toolchain/base/int.h
index c1b2f9affe81..c75d7bdd798a 100644
--- a/toolchain/base/int.h
+++ b/toolchain/base/int.h
@@ -206,12 +206,12 @@ class IntId : public Printable<IntId> {
   int32_t id_;
 };
 
-constexpr IntId IntId::None(IntId::NoneId);
+inline constexpr IntId IntId::None(IntId::NoneId);
 
 // Note that we initialize the `None` index in a constexpr context which
 // ensures there is no UB in forming it. This helps ensure all the ID -> index
 // conversions are correct because the `None` ID is at the limit of that range.
-constexpr int32_t IntId::NoneIndex = None.AsIndex();
+inline constexpr int32_t IntId::NoneIndex = None.AsIndex();
 
 // A canonicalizing value store with deep optimizations for integers.
 //
@@ -428,7 +428,8 @@ class IntStore {
   CanonicalValueStore<APIntId, llvm::APInt> values_;
 };
 
-constexpr IntStore::APIntId IntStore::APIntId::None(IntId::None.AsIndex());
+inline constexpr IntStore::APIntId IntStore::APIntId::None(
+    IntId::None.AsIndex());
 
 }  // namespace Carbon
 
diff --git a/toolchain/base/llvm_tools.h b/toolchain/base/llvm_tools.h
index fe05158e9eb0..e82178ce5187 100644
--- a/toolchain/base/llvm_tools.h
+++ b/toolchain/base/llvm_tools.h
@@ -63,12 +63,12 @@ class LLVMTool : public CARBON_ENUM_BASE(LLVMTool) {
   CARBON_ENUM_CONSTANT_DEFINITION(LLVMTool, Identifier)
 #include "toolchain/base/llvm_tools.def"
 
-constexpr LLVMTool LLVMTool::ToolsStorage[] = {
+inline constexpr LLVMTool LLVMTool::ToolsStorage[] = {
 #define CARBON_LLVM_TOOL(Identifier, Name, BinName, MainFn) \
   LLVMTool::Identifier,
 #include "toolchain/base/llvm_tools.def"
 };
-constexpr llvm::ArrayRef<LLVMTool> LLVMTool::Tools = ToolsStorage;
+inline constexpr llvm::ArrayRef<LLVMTool> LLVMTool::Tools = ToolsStorage;
 
 }  // namespace Carbon
 
diff --git a/toolchain/base/runtime_sources.bzl b/toolchain/base/runtime_sources.bzl
index 34d6a7201755..4212cfc5044a 100644
--- a/toolchain/base/runtime_sources.bzl
+++ b/toolchain/base/runtime_sources.bzl
@@ -54,34 +54,34 @@ _TEMPLATE = """
 
 namespace Carbon::RuntimeSources {{
 
-constexpr inline llvm::StringLiteral CrtBegin = {crtbegin_src};
-constexpr inline llvm::StringLiteral CrtEnd = {crtend_src};
+inline constexpr llvm::StringLiteral CrtBegin = {crtbegin_src};
+inline constexpr llvm::StringLiteral CrtEnd = {crtend_src};
 
-constexpr inline llvm::StringLiteral BuiltinsGenericSrcs[] = {{
+inline constexpr llvm::StringLiteral BuiltinsGenericSrcs[] = {{
 {generic_srcs}
 }};
-constexpr inline llvm::StringLiteral BuiltinsMacosSrcs[] = {{
+inline constexpr llvm::StringLiteral BuiltinsMacosSrcs[] = {{
 {macos_srcs}
 }};
-constexpr inline llvm::StringLiteral BuiltinsBf16Srcs[] = {{
+inline constexpr llvm::StringLiteral BuiltinsBf16Srcs[] = {{
 {bf16_srcs}
 }};
-constexpr inline llvm::StringLiteral BuiltinsTfSrcs[] = {{
+inline constexpr llvm::StringLiteral BuiltinsTfSrcs[] = {{
 {tf_srcs}
 }};
-constexpr inline llvm::StringLiteral BuiltinsX86ArchSrcs[] = {{
+inline constexpr llvm::StringLiteral BuiltinsX86ArchSrcs[] = {{
 {x86_arch_srcs}
 }};
-constexpr inline llvm::StringLiteral BuiltinsX86Fp80Srcs[] = {{
+inline constexpr llvm::StringLiteral BuiltinsX86Fp80Srcs[] = {{
 {x86_fp80_srcs}
 }};
-constexpr inline llvm::StringLiteral BuiltinsAarch64Srcs[] = {{
+inline constexpr llvm::StringLiteral BuiltinsAarch64Srcs[] = {{
 {aarch64_srcs}
 }};
-constexpr inline llvm::StringLiteral BuiltinsX86_64Srcs[] = {{
+inline constexpr llvm::StringLiteral BuiltinsX86_64Srcs[] = {{
 {x86_64_srcs}
 }};
-constexpr inline llvm::StringLiteral BuiltinsI386Srcs[] = {{
+inline constexpr llvm::StringLiteral BuiltinsI386Srcs[] = {{
 {i386_srcs}
 }};
 
diff --git a/toolchain/base/value_ids.h b/toolchain/base/value_ids.h
index e3c90a60df58..37369218cf16 100644
--- a/toolchain/base/value_ids.h
+++ b/toolchain/base/value_ids.h
@@ -49,7 +49,7 @@ struct FloatId : public IdBase<FloatId> {
   static const FloatId None;
   using IdBase::IdBase;
 };
-constexpr FloatId FloatId::None(FloatId::NoneIndex);
+inline constexpr FloatId FloatId::None(FloatId::NoneIndex);
 
 // Corresponds to a Real value.
 struct RealId : public IdBase<RealId> {
@@ -62,7 +62,7 @@ struct RealId : public IdBase<RealId> {
   static const RealId None;
   using IdBase::IdBase;
 };
-constexpr RealId RealId::None(RealId::NoneIndex);
+inline constexpr RealId RealId::None(RealId::NoneIndex);
 
 // Corresponds to StringRefs for identifiers.
 //
@@ -73,7 +73,7 @@ struct IdentifierId : public IdBase<IdentifierId> {
   static const IdentifierId None;
   using IdBase::IdBase;
 };
-constexpr IdentifierId IdentifierId::None(IdentifierId::NoneIndex);
+inline constexpr IdentifierId IdentifierId::None(IdentifierId::NoneIndex);
 
 // The name of a package, which is either an identifier or the special `Core`
 // package name.
@@ -122,9 +122,10 @@ struct PackageNameId : public IdBase<PackageNameId> {
     }
   }
 };
-constexpr PackageNameId PackageNameId::None(PackageNameId::NoneIndex);
-constexpr PackageNameId PackageNameId::Core(PackageNameId::NoneIndex - 1);
-constexpr PackageNameId PackageNameId::Cpp(PackageNameId::NoneIndex - 2);
+inline constexpr PackageNameId PackageNameId::None(PackageNameId::NoneIndex);
+inline constexpr PackageNameId PackageNameId::Core(PackageNameId::NoneIndex -
+                                                   1);
+inline constexpr PackageNameId PackageNameId::Cpp(PackageNameId::NoneIndex - 2);
 
 // Corresponds to StringRefs for string literals.
 struct StringLiteralValueId : public IdBase<StringLiteralValueId> {
@@ -132,7 +133,7 @@ struct StringLiteralValueId : public IdBase<StringLiteralValueId> {
   static const StringLiteralValueId None;
   using IdBase::IdBase;
 };
-constexpr StringLiteralValueId StringLiteralValueId::None(
+inline constexpr StringLiteralValueId StringLiteralValueId::None(
     StringLiteralValueId::NoneIndex);
 
 }  // namespace Carbon
diff --git a/toolchain/check/convert.h b/toolchain/check/convert.h
index 254863a37620..1a7a4ab29b4e 100644
--- a/toolchain/check/convert.h
+++ b/toolchain/check/convert.h
@@ -149,7 +149,7 @@ struct TypeExpr {
   SemIR::TypeId type_id;
 };
 
-constexpr inline TypeExpr TypeExpr::None = {.inst_id = SemIR::TypeInstId::None,
+inline constexpr TypeExpr TypeExpr::None = {.inst_id = SemIR::TypeInstId::None,
                                             .type_id = SemIR::TypeId::None};
 
 // Converts an expression for use as a type.
diff --git a/toolchain/check/eval_inst.h b/toolchain/check/eval_inst.h
index 5c54b0ee3174..3d878d1dd773 100644
--- a/toolchain/check/eval_inst.h
+++ b/toolchain/check/eval_inst.h
@@ -86,13 +86,13 @@ class ConstantEvalResult {
   bool same_phase_as_inst_;
 };
 
-constexpr ConstantEvalResult ConstantEvalResult::Error =
+inline constexpr ConstantEvalResult ConstantEvalResult::Error =
     Existing(SemIR::ErrorInst::ConstantId);
 
-constexpr ConstantEvalResult ConstantEvalResult::NotConstant =
+inline constexpr ConstantEvalResult ConstantEvalResult::NotConstant =
     ConstantEvalResult(SemIR::ConstantId::NotConstant);
 
-constexpr ConstantEvalResult ConstantEvalResult::TODO = NotConstant;
+inline constexpr ConstantEvalResult ConstantEvalResult::TODO = NotConstant;
 
 // Implementation details to compute the type of the `EvalConstantInst`
 // functions.
diff --git a/toolchain/check/keyword_modifier_set.h b/toolchain/check/keyword_modifier_set.h
index a8c4f39196ae..88d13d5da35f 100644
--- a/toolchain/check/keyword_modifier_set.h
+++ b/toolchain/check/keyword_modifier_set.h
@@ -112,15 +112,19 @@ class KeywordModifierSet : public CARBON_ENUM_MASK_BASE(KeywordModifierSet) {
 CARBON_KEYWORD_MODIFIER_SET(CARBON_KEYWORD_MODIFIER_SET_WITH_TYPE)
 #undef CARBON_KEYWORD_MODIFIER_SET_WITH_TYPE
 
-constexpr KeywordModifierSet KeywordModifierSet::Access(Private | Protected);
-constexpr KeywordModifierSet KeywordModifierSet::Class(Abstract | Base);
-constexpr KeywordModifierSet KeywordModifierSet::Method(Abstract | Override |
-                                                        Virtual);
-constexpr KeywordModifierSet KeywordModifierSet::ImplDecl(Extend | Final);
-constexpr KeywordModifierSet KeywordModifierSet::Interface(Default | Final);
-constexpr KeywordModifierSet KeywordModifierSet::Decl(Class | Method | Impl |
-                                                      Interface | Export |
-                                                      Returned);
+inline constexpr KeywordModifierSet KeywordModifierSet::Access(Private |
+                                                               Protected);
+inline constexpr KeywordModifierSet KeywordModifierSet::Class(Abstract | Base);
+inline constexpr KeywordModifierSet KeywordModifierSet::Method(Abstract |
+                                                               Override |
+                                                               Virtual);
+inline constexpr KeywordModifierSet KeywordModifierSet::ImplDecl(Extend |
+                                                                 Final);
+inline constexpr KeywordModifierSet KeywordModifierSet::Interface(Default |
+                                                                  Final);
+inline constexpr KeywordModifierSet KeywordModifierSet::Decl(Class | Method |
+                                                             Impl | Interface |
+                                                             Export | Returned);
 
 static_assert(
     !KeywordModifierSet::Access.HasAnyOf(KeywordModifierSet::Extern) &&
diff --git a/toolchain/check/node_stack.h b/toolchain/check/node_stack.h
index b783c3cce076..e1659c14311a 100644
--- a/toolchain/check/node_stack.h
+++ b/toolchain/check/node_stack.h
@@ -637,7 +637,7 @@ class NodeStack {
   llvm::SmallVector<Entry> stack_;
 };
 
-constexpr NodeStack::IdKindTableType NodeStack::IdKindTable =
+inline constexpr NodeStack::IdKindTableType NodeStack::IdKindTable =
     ComputeIdKindTable();
 
 inline auto NodeStack::PopExprWithNodeId()
diff --git a/toolchain/check/scope_index.h b/toolchain/check/scope_index.h
index d8d71b5cd370..09154eb9fbe0 100644
--- a/toolchain/check/scope_index.h
+++ b/toolchain/check/scope_index.h
@@ -24,7 +24,7 @@ struct ScopeIndex : public IndexBase<ScopeIndex> {
   using IndexBase::IndexBase;
 };
 
-constexpr ScopeIndex ScopeIndex::Package = ScopeIndex(0);
+inline constexpr ScopeIndex ScopeIndex::Package = ScopeIndex(0);
 
 }  // namespace Carbon::Check
 
diff --git a/toolchain/lex/token_index.h b/toolchain/lex/token_index.h
index 8536df5375b9..2a4a7a30b768 100644
--- a/toolchain/lex/token_index.h
+++ b/toolchain/lex/token_index.h
@@ -40,8 +40,8 @@ struct TokenIndex : public IndexBase<TokenIndex> {
   using IndexBase::IndexBase;
 };
 
-constexpr TokenIndex TokenIndex::None(TokenIndex::NoneIndex);
-constexpr TokenIndex TokenIndex::FirstNonCommentToken(1);
+inline constexpr TokenIndex TokenIndex::None(TokenIndex::NoneIndex);
+inline constexpr TokenIndex TokenIndex::FirstNonCommentToken(1);
 
 // A lightweight handle to a lexed token in a `TokenizedBuffer` whose kind is
 // known to be `Kind`.
diff --git a/toolchain/lex/token_kind.h b/toolchain/lex/token_kind.h
index bfe1a49fcebf..352a5ae3bf0e 100644
--- a/toolchain/lex/token_kind.h
+++ b/toolchain/lex/token_kind.h
@@ -130,11 +130,11 @@ class TokenKind : public CARBON_ENUM_BASE(TokenKind) {
   CARBON_ENUM_CONSTANT_DEFINITION(TokenKind, TokenName)
 #include "toolchain/lex/token_kind.def"
 
-constexpr TokenKind TokenKind::KeywordTokensStorage[] = {
+inline constexpr TokenKind TokenKind::KeywordTokensStorage[] = {
 #define CARBON_KEYWORD_TOKEN(TokenName, Spelling) TokenKind::TokenName,
 #include "toolchain/lex/token_kind.def"
 };
-constexpr llvm::ArrayRef<TokenKind> TokenKind::KeywordTokens =
+inline constexpr llvm::ArrayRef<TokenKind> TokenKind::KeywordTokens =
     KeywordTokensStorage;
 
 }  // namespace Carbon::Lex
diff --git a/toolchain/lex/tokenized_buffer.h b/toolchain/lex/tokenized_buffer.h
index 75a8d5188d71..5484c9a1b2f2 100644
--- a/toolchain/lex/tokenized_buffer.h
+++ b/toolchain/lex/tokenized_buffer.h
@@ -56,7 +56,7 @@ struct LineIndex : public IndexBase<LineIndex> {
   using IndexBase::IndexBase;
 };
 
-constexpr LineIndex LineIndex::None(NoneIndex);
+inline constexpr LineIndex LineIndex::None(NoneIndex);
 
 // A comment, which can be a block of lines. These are tracked separately from
 // tokens because they don't affect parse; if they were part of tokens, we'd
@@ -79,7 +79,7 @@ struct CommentIndex : public IndexBase<CommentIndex> {
   using IndexBase::IndexBase;
 };
 
-constexpr CommentIndex CommentIndex::None(NoneIndex);
+inline constexpr CommentIndex CommentIndex::None(NoneIndex);
 
 // Random-access iterator over comments within the buffer.
 using CommentIterator = IndexIterator<CommentIndex>;
diff --git a/toolchain/parse/node_kind.h b/toolchain/parse/node_kind.h
index cee4259f9891..682ed9ce68a4 100644
--- a/toolchain/parse/node_kind.h
+++ b/toolchain/parse/node_kind.h
@@ -80,7 +80,7 @@ class NodeKind : public CARBON_ENUM_BASE(NodeKind) {
   CARBON_ENUM_CONSTANT_DEFINITION(NodeKind, Name)
 #include "toolchain/parse/node_kind.def"
 
-constexpr int NodeKind::ValidCount = 0
+inline constexpr int NodeKind::ValidCount = 0
 #define CARBON_PARSE_NODE_KIND(Name) +1
 #include "toolchain/parse/node_kind.def"
     ;
diff --git a/toolchain/sem_ir/ids.h b/toolchain/sem_ir/ids.h
index 2758b1064238..3ee4796f08df 100644
--- a/toolchain/sem_ir/ids.h
+++ b/toolchain/sem_ir/ids.h
@@ -42,8 +42,9 @@ struct InstId : public IdBase<InstId> {
   auto Print(llvm::raw_ostream& out) const -> void;
 };
 
-constexpr InstId InstId::InitTombstone = InstId(NoneIndex - 1);
-constexpr InstId InstId::ImplWitnessTablePlaceholder = InstId(NoneIndex - 2);
+inline constexpr InstId InstId::InitTombstone = InstId(NoneIndex - 1);
+inline constexpr InstId InstId::ImplWitnessTablePlaceholder =
+    InstId(NoneIndex - 2);
 
 // An InstId whose value is a type. The fact it's a type must be validated
 // before construction, and this allows that validation to be represented in the
@@ -63,7 +64,8 @@ struct TypeInstId : public InstId {
       : InstId(id) {}
 };
 
-constexpr TypeInstId TypeInstId::None = TypeInstId::UnsafeMake(InstId::None);
+inline constexpr TypeInstId TypeInstId::None =
+    TypeInstId::UnsafeMake(InstId::None);
 
 // An InstId whose type is known to be T. The fact it's a type must be validated
 // before construction, and this allows that validation to be represented in the
@@ -89,7 +91,7 @@ struct KnownInstId : public InstId {
 };
 
 template <typename T>
-constexpr KnownInstId<T> KnownInstId<T>::None =
+inline constexpr KnownInstId<T> KnownInstId<T>::None =
     KnownInstId<T>::UnsafeMake(InstId::None);
 
 // An ID of an instruction that is referenced absolutely by another instruction.
@@ -249,7 +251,8 @@ struct ConstantId : public IdBase<ConstantId> {
   static constexpr int32_t FirstSymbolicId = NoneIndex - 2;
 };
 
-constexpr ConstantId ConstantId::NotConstant = ConstantId(NotConstantIndex);
+inline constexpr ConstantId ConstantId::NotConstant =
+    ConstantId(NotConstantIndex);
 
 // The ID of a `EntityName`.
 struct EntityNameId : public IdBase<EntityNameId> {
@@ -311,7 +314,7 @@ struct CheckIRId : public IdBase<CheckIRId> {
   auto Print(llvm::raw_ostream& out) const -> void;
 };
 
-constexpr CheckIRId CheckIRId::Cpp = CheckIRId(NoneIndex - 1);
+inline constexpr CheckIRId CheckIRId::Cpp = CheckIRId(NoneIndex - 1);
 
 // The ID of a `Class`.
 struct ClassId : public IdBase<ClassId> {
@@ -444,7 +447,7 @@ struct GenericInstIndex : public IndexBase<GenericInstIndex> {
   static constexpr int32_t FirstDefinitionIndex = NoneIndex - 1;
 };
 
-constexpr GenericInstIndex GenericInstIndex::None =
+inline constexpr GenericInstIndex GenericInstIndex::None =
     GenericInstIndex::MakeNone();
 
 // The ID of an `ImportIR` within the set of imported IRs, both direct and
@@ -465,8 +468,8 @@ struct ImportIRId : public IdBase<ImportIRId> {
   auto Print(llvm::raw_ostream& out) const -> void;
 };
 
-constexpr ImportIRId ImportIRId::ApiForImpl = ImportIRId(0);
-constexpr ImportIRId ImportIRId::Cpp = ImportIRId(ApiForImpl.index + 1);
+inline constexpr ImportIRId ImportIRId::ApiForImpl = ImportIRId(0);
+inline constexpr ImportIRId ImportIRId::Cpp = ImportIRId(ApiForImpl.index + 1);
 
 // A boolean value.
 struct BoolValue : public IdBase<BoolValue> {
@@ -490,8 +493,8 @@ struct BoolValue : public IdBase<BoolValue> {
   auto Print(llvm::raw_ostream& out) const -> void;
 };
 
-constexpr BoolValue BoolValue::False = BoolValue(0);
-constexpr BoolValue BoolValue::True = BoolValue(1);
+inline constexpr BoolValue BoolValue::False = BoolValue(0);
+inline constexpr BoolValue BoolValue::True = BoolValue(1);
 
 // A character literal value as a unicode codepoint.
 struct CharId : public IdBase<CharId> {
@@ -521,8 +524,8 @@ struct IntKind : public IdBase<IntKind> {
   auto Print(llvm::raw_ostream& out) const -> void;
 };
 
-constexpr IntKind IntKind::Unsigned = IntKind(0);
-constexpr IntKind IntKind::Signed = IntKind(1);
+inline constexpr IntKind IntKind::Unsigned = IntKind(0);
+inline constexpr IntKind IntKind::Signed = IntKind(1);
 
 // A float kind value. This describes the semantics of the floating-point type.
 // This represents very similar information to the bit-width, but is more
@@ -560,15 +563,15 @@ struct FloatKind : public IdBase<FloatKind> {
   auto Semantics() const -> const llvm::fltSemantics&;
 };
 
-constexpr FloatKind FloatKind::None = FloatKind(NoneIndex);
+inline constexpr FloatKind FloatKind::None = FloatKind(NoneIndex);
 
-constexpr FloatKind FloatKind::Binary16 = FloatKind(0);
-constexpr FloatKind FloatKind::Binary32 = FloatKind(1);
-constexpr FloatKind FloatKind::Binary64 = FloatKind(2);
-constexpr FloatKind FloatKind::Binary128 = FloatKind(3);
-constexpr FloatKind FloatKind::BFloat16 = FloatKind(4);
-constexpr FloatKind FloatKind::X87Float80 = FloatKind(5);
-constexpr FloatKind FloatKind::PPCFloat128 = FloatKind(6);
+inline constexpr FloatKind FloatKind::Binary16 = FloatKind(0);
+inline constexpr FloatKind FloatKind::Binary32 = FloatKind(1);
+inline constexpr FloatKind FloatKind::Binary64 = FloatKind(2);
+inline constexpr FloatKind FloatKind::Binary128 = FloatKind(3);
+inline constexpr FloatKind FloatKind::BFloat16 = FloatKind(4);
+inline constexpr FloatKind FloatKind::X87Float80 = FloatKind(5);
+inline constexpr FloatKind FloatKind::PPCFloat128 = FloatKind(6);
 
 // An X-macro for special names. Uses should look like:
 //
@@ -654,14 +657,14 @@ struct NameId : public IdBase<NameId> {
 
 // Define the special `static const NameId` values.
 #define CARBON_SPECIAL_NAME_ID_FOR_DEF(Name) \
-  constexpr NameId NameId::Name =            \
+  inline constexpr NameId NameId::Name =     \
       NameId(NoneIndex - 1 - static_cast<int>(NameId::SpecialNameId::Name));
 CARBON_SPECIAL_NAME_ID(CARBON_SPECIAL_NAME_ID_FOR_DEF)
 #undef CARBON_SPECIAL_NAME_ID_FOR_DEF
 
 // Count non-index values, including `None` and special names.
 #define CARBON_SPECIAL_NAME_ID_FOR_COUNT(...) +1
-constexpr int NameId::NonIndexValueCount =
+inline constexpr int NameId::NonIndexValueCount =
     1 CARBON_SPECIAL_NAME_ID(CARBON_SPECIAL_NAME_ID_FOR_COUNT);
 #undef CARBON_SPECIAL_NAME_ID_FOR_COUNT
 
@@ -675,7 +678,7 @@ struct NameScopeId : public IdBase<NameScopeId> {
   using IdBase::IdBase;
 };
 
-constexpr NameScopeId NameScopeId::Package = NameScopeId(0);
+inline constexpr NameScopeId NameScopeId::Package = NameScopeId(0);
 
 // The ID of an `InstId` block.
 struct InstBlockId : public IdBase<InstBlockId> {
@@ -705,11 +708,12 @@ struct InstBlockId : public IdBase<InstBlockId> {
   auto Print(llvm::raw_ostream& out) const -> void;
 };
 
-constexpr InstBlockId InstBlockId::Empty = InstBlockId(0);
-constexpr InstBlockId InstBlockId::Exports = InstBlockId(1);
-constexpr InstBlockId InstBlockId::Imports = InstBlockId(2);
-constexpr InstBlockId InstBlockId::GlobalInit = InstBlockId(3);
-constexpr InstBlockId InstBlockId::Unreachable = InstBlockId(NoneIndex - 1);
+inline constexpr InstBlockId InstBlockId::Empty = InstBlockId(0);
+inline constexpr InstBlockId InstBlockId::Exports = InstBlockId(1);
+inline constexpr InstBlockId InstBlockId::Imports = InstBlockId(2);
+inline constexpr InstBlockId InstBlockId::GlobalInit = InstBlockId(3);
+inline constexpr InstBlockId InstBlockId::Unreachable =
+    InstBlockId(NoneIndex - 1);
 
 // Contains either an `InstBlockId` value, an error value, or
 // `InstBlockId::None`.
@@ -817,7 +821,8 @@ struct StructTypeFieldsId : public IdBase<StructTypeFieldsId> {
   using IdBase::IdBase;
 };
 
-constexpr StructTypeFieldsId StructTypeFieldsId::Empty = StructTypeFieldsId(0);
+inline constexpr StructTypeFieldsId StructTypeFieldsId::Empty =
+    StructTypeFieldsId(0);
 
 // The ID of a `CustomLayout` block.
 struct CustomLayoutId : public IdBase<CustomLayoutId> {
@@ -837,7 +842,7 @@ struct CustomLayoutId : public IdBase<CustomLayoutId> {
   using IdBase::IdBase;
 };
 
-constexpr CustomLayoutId CustomLayoutId::Empty = CustomLayoutId(0);
+inline constexpr CustomLayoutId CustomLayoutId::Empty = CustomLayoutId(0);
 
 // The ID of a type.
 struct TypeId : public IdBase<TypeId> {
@@ -906,8 +911,10 @@ struct LibraryNameId : public IdBase<LibraryNameId> {
   auto Print(llvm::raw_ostream& out) const -> void;
 };
 
-constexpr LibraryNameId LibraryNameId::Default = LibraryNameId(NoneIndex - 1);
-constexpr LibraryNameId LibraryNameId::Error = LibraryNameId(NoneIndex - 2);
+inline constexpr LibraryNameId LibraryNameId::Default =
+    LibraryNameId(NoneIndex - 1);
+inline constexpr LibraryNameId LibraryNameId::Error =
+    LibraryNameId(NoneIndex - 2);
 
 // The ID of an `ImportIRInst`.
 struct ImportIRInstId : public IdBase<ImportIRInstId> {
@@ -940,7 +947,7 @@ struct RequireImplsBlockId : public IdBase<RequireImplsBlockId> {
   using IdBase::IdBase;
 };
 
-constexpr RequireImplsBlockId RequireImplsBlockId::Empty =
+inline constexpr RequireImplsBlockId RequireImplsBlockId::Empty =
     RequireImplsBlockId(0);
 
 // A SemIR location used as the location of instructions. This contains either a
diff --git a/toolchain/sem_ir/impl.h b/toolchain/sem_ir/impl.h
index 49e38f142963..e4b2039b96b1 100644
--- a/toolchain/sem_ir/impl.h
+++ b/toolchain/sem_ir/impl.h
@@ -214,7 +214,7 @@ class ImplStore {
   llvm::SmallVector<llvm::SmallVector<ImplId, 2>> lookup_buckets_;
 };
 
-constexpr inline ImplStore::ImplOrLookupBucketId
+inline constexpr ImplStore::ImplOrLookupBucketId
     ImplStore::ImplOrLookupBucketId::None(NoneIndex);
 
 }  // namespace Carbon::SemIR