[202x][0015] Implement mutable keyword - #9007

Mutable fields cause unsafe static promotion of const locals

tools/​clang/​lib/​Parse/​ParseDecl.cpp:3957

Enabling mutable makes the local-const promotion in CGDecl.cpp:145–152 unsafe. For struct M { mutable int value; };, the local in int f() { const M m = {0}; return ++m.value; } takes the EmitStaticVarDecl path. Repeated calls then share storage instead of reinitializing m on each call. Exclude types containing mutable fields from this promotion, using the existing CGM.isTypeConstant check where appropriate, and add a regression that calls the function twice and expects 1 both times.

Read-only resource validation misses mutable fields in new specializations

tools/​clang/​lib/​Sema/​SemaHLSL.cpp:5926

The read-only restriction misses previously uninstantiated element specializations, such as template<class T> struct Data { mutable T value; }; StructuredBuffer<Data<int>> buf;. Unlike the ConstantBuffer branch, this path does not require a complete element type before traversal, so DiagnoseElementTypes sees no fields and accepts it. Later global validation stops at the resource object rather than revisiting its element. Complete non-dependent element types before walking their fields, or defer validation until completion, and add a regression where the buffer is the specialization's first use.

Const methods lose writes to mutable receiver fields

tools/​clang/​test/​HLSLFileCheck/​hlsl/​classes/​const_method_202x_codegen.hlsl:13

Mutable writes do not survive this const-method call. CGHLSLMS.cpp:1918–1921 marks every const method's this as In, so HLLegalizeParameter::patchWriteOnInParam redirects stores to a temporary without copying them back before inlining. With cache initialized to 0, ls.incrementCache(); return ls.cache; therefore returns 0 instead of 1. Preserve write-back for receivers containing mutable fields, including nested and base-class fields, and extend this test to observe the receiver after the call. Checking only the method's return value hides the lost update.