[analyzer] Fix non decimal macro values in tryExpandAsInteger (#168632)

Values were parsed into an unsigned APInt with just enough of a bit
width to hold the number then interpreted as signed values. This
resulted in hex, octal and binary literals from being interpreted as
negative when the most significant bit is 1.

For example the `-0b11` would have a bit width of 2, would be
interpreted as -1, then negated to become 1.

Author: ColinKinloch

clang/lib/StaticAnalyzer/Core/CheckerHelpers.cpp

@@ -142,19 +142,25 @@ std::optional<int> tryExpandAsInteger(StringRef Macro, const Preprocessor &PP) {
   if (InvalidSpelling)
     return std::nullopt;
 
-  llvm::APInt IntValue;
+  llvm::APSInt IntValue(/*BitWidth=*/0, /*isUnsigned=*/true);
   constexpr unsigned AutoSenseRadix = 0;
-  if (ValueStr.getAsInteger(AutoSenseRadix, IntValue))
+  if (ValueStr.getAsInteger(AutoSenseRadix,
+                            static_cast<llvm::APInt &>(IntValue)))
     return std::nullopt;
 
   // Parse an optional minus sign.
   size_t Size = FilteredTokens.size();
   if (Size >= 2) {
-    if (FilteredTokens[Size - 2].is(tok::minus))
+    if (FilteredTokens[Size - 2].is(tok::minus)) {
+      // Make sure there's space for a sign bit
+      if (IntValue.isSignBitSet())
+        IntValue = IntValue.extend(IntValue.getBitWidth() + 1);
+      IntValue.setIsUnsigned(false);
       IntValue = -IntValue;
+    }
   }
 
-  return IntValue.getSExtValue();
+  return IntValue.getExtValue();
 }
 
 OperatorKind operationKindFromOverloadedOperator(OverloadedOperatorKind OOK,

clang/test/Analysis/std-c-library-functions-eof-2-rad.c

@@ -0,0 +1,16 @@
+// RUN: %clang_analyze_cc1 -std=c23 -analyzer-checker=core,unix.StdCLibraryFunctions,debug.ExprInspection -verify -analyzer-config eagerly-assume=false %s
+
+void clang_analyzer_eval(int);
+
+typedef struct FILE FILE;
+/// Test that the static analyzer doesn't interpret the most significant bit as the sign bit.
+// Unorthodox EOF value with a power of 2 radix
+#define EOF (-0b11)
+
+int getc(FILE *);
+void test_getc(FILE *fp) {
+  int y = getc(fp);
+  if (y < 0) {
+    clang_analyzer_eval(y == EOF); // expected-warning{{TRUE}}
+  }
+}